1. Field of the Invention
The present invention relates to a variable gain amplifier circuit and gain control method and, more particularly, to a variable gain amplifier circuit and gain control method which are used for the receiving section of a portable telephone terminal.
2. Description of the Prior Art
Recently, the portable telephone market has rapidly developed. In the receiving section of a portable telephone terminal, a weak RF signal received by an antenna is input first to a low-noise amplifier circuit to be amplified to a desired power, and then, only a desired frequency band is selected by a filter. The resultant signal is frequency-converted into an IF signal.
In a currently used digital portable telephone, to ensure a predetermined bit error rate, a circuit used for a receiving section is required to have a wide range of linearity because signals having various strengths can be received depending on the distance between the receiving section and a base station.
In order to ensure the linear operation of the circuit connected to the subsequent stage of the reception circuit, the above low-noise amplifier circuit is required to have a gain changing function by operating at a high gain with respect to a weak signal and decreasing the gain upon reception of a strong signal.
When a silicon bipolar transistor is used for this variable gain type low-noise amplifier circuit as in conventional terminals, the circuit uses two or more amplifiers and has a gain changing function after the second amplifier.
A heterojunction bipolar transistor (to be referred to as an HBT hereinafter), using a compound semiconductor such as GaAs, can be used for a portable telephone terminal as well.
This compound HBT has excellent RF characteristics, and hence a portable telephone low-noise amplifier circuit can be formed using only one amplifier. This component characteristic can decrease the number of elements used in constructing a portable telephone terminal. It is therefore expected that the reduction in number of components permits reduction in the area occupied by the circuit and power consumption.
Consider the circuit shown in FIG. 1 as a single-stage variable gain amplifier circuit using a bipolar transistor such as the compound HBT.
FIG. 1 is a circuit diagram showing a conventional variable gain amplifier circuit. This variable gain amplifier circuit is comprised of an npn transistor 51, a capacitor 52 and inductor 53 which are connected to the collector of the npn transistor 51, a capacitor 54 and inductor 55 which are connected to the base of the npn transistor 51, bias application terminals 56 and 57 each connected to the other terminal of a corresponding one of the inductors 53 and 55, an output terminal 58 connected to the other terminal of the capacitor 52, and an input terminal 59 connected to the other terminal of the capacitor 54.
This conventional circuit is an emitter-grounded amplifier circuit using the transistor 51, in which DC biases are applied to the base and collector of the transistor through the bias application terminals 56 and 57 and inductors 53 and 55, an input signal input through the input terminal 59 is input to the base of the transistor 51 through the DC blocking capacitor 54 current, and an output signal output from the collector of the transistor 51 is output from the output terminal 58 through the DC blocking capacitor 52.
In general, an LC impedance matching circuit is often connected to the input/output section, although this circuit is omitted in this case.
In a bipolar transistor, the transconductance (gm) of the elements changes in accordance with the base bias, and the gain approximates gmxc2x7ZL where ZL is the load impedance. Therefore, by using the bias application terminal 57 as a gain control terminal and changing the voltage to be applied, this circuit can be used as a variable gain amplifier circuit.
Other variable gain amplifier circuits of this type are disclosed in Japanese Unexamined Patent Publication Nos. 10-93390, 62-188411, 57-147309, and 57-132410 (to be referred to as references 1 to 4, respectively, hereinafter).
The problem in the conventional variable gain amplifier circuit shown in FIG. 1 is that a third-order input intercept point (to be referred to as an IIP3 hereinafter) decreases with a decrease in gain, as indicated by the graph shown in FIG. 2 which shows the gain dependence of the IIP3 of the conventional circuit.
This is because, when a low base bias is set for a low gain, the difference between the base bias and the base ON voltage becomes small which increases the nonlinearity of the current/voltage characteristics between the base and the emitter, thereby readily distorting the input waveform.
In the receiving section of a portable telephone, low-gain operation is performed upon reception of a strong signal, and hence a larger IIP3 is required in low-gain operation than in high-gain operation. For this reason, in order to apply this circuit to a portable telephone, the IIP3 must be increased in low-gain operation.
Japanese Unexamined Patent Publication No. 6-78241 (to be referred to as reference 5 hereinafter) discloses an automatic gain control circuit for controlling the gain of a variable gain amplifier 5 by using an emitter follower 20.
In this circuit, however, since the base bias voltage of the variable gain amplifier 5 is controlled by the emitter follower 20, the IIP3 undesirably decreases as in the conventional circuit described above.
The automatic gain control circuit in reference 5 is used for a satellite broadcasting receiver. In this regard as well, this conventional circuit differs from the present invention.
The present invention has been made in consideration of the above drawbacks of the prior art, and has as its object to provide a variable amplifier circuit and gain control method which can increase IIP3 in low-gain operation.
In order to achieve the above object, according to the first aspect of the present invention, there is provided a variable amplifier circuit comprising signal amplification means and variable impedance means connected to an input section of the signal amplification means and having a variable output impedance.
According to the first aspect, gain control is performed by the fact that the variable impedance means changes the input impedance of the signal amplification means. That is, gain control is performed without changing the input-side bias voltage of the signal amplification means, and hence the IIP3 can be increased in low-gain operation. In other words, linearity is maintained even in low-gain operation.
This arrangement permits linearity to be maintained because an emitter follower is connected in parallel with the input terminal of an emitter-grounded amplifier, and the emitter follower is used to control the power input to the emitter-grounded amplifier circuit so as to prevent a deterioration in the distortion characteristics of the emitter-grounded amplifier circuit.
According to the second aspect of the present invention, there is provided a gain control method for a variable gain amplifier circuit including signal amplification means and variable impedance means connected to an input section of the signal amplification means and having a variable output impedance, comprising the first step of keeping a bias voltage applied to the input section of the signal amplification means constant, and the second step of changing an output impedance of the variable impedance means in a direction in which a gain of the signal amplification means decreases.
The second aspect has the same effects as those of the first aspect.
In the present invention, an emitter-grounded amplifier circuit is used as the signal amplification means, and an emitter follower is used as the variable impedance means.
The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principles of the present invention are shown by way of illustrative examples.